"Couldn't?" I think you just said the opposite of what you meant, and ended up being correct. See https://news.ycombinator.com/item?id=15257004 where I lay out the actual logic involved.
This is what had me questioning whether an implicit licence could be revoked:
There is no precedent for BSD/MIT implying any patent grant.
So in other words it's only the chance of an implicit patent grant, which may or may not actually hold up.
In fact, this entire argument is backwards. React is licensed under the BSD license and comes with an additional patent grant. If the BSD license would imply any patent grant, the additional patent grant would only count on top of that so the FB patent grant would actually be irrelevant. If that's not the case, there is evidently no implicit patent grant in the BSD license. You can't have it both ways.
(e.g. many parts of the GPL have never been tested in court.)